Pleochroic dye molecules have the property that the amount of light that they absorb depends on their orientation to the electric vector of the incident radiation. That is, the absorption of light by the dye molecules is maximized when the electric vector of the light is parallel to the transition axes of them, and is minimized when the electric vector of the light is perpendicular to the axes of them.
In the aforesaid liquid crystal display device, orientation of the dye molecules can be changed by utilizing the phenomenon that the orientation direction of liquid crystal molecules is changed by applying a voltage. Thus, the amount of light that dye molecules absorb in such liquid crystal display device can be changed by applying a voltage. This phenomenon is called "guest-host effect" taking the liquid crystal compound as host and the dye as guest.
Of the pleochroic dyes, those which have parallel dichroism show such properties that the direction of transition axis of visible light absorption is almost parallel to the longer axis of the molecule and, when dissolved as a guest molecule in a host liquid crystal material, the dye molecules are aligned so that the longer axis of the dye molecule becomes to be parallel to the orientation direction of liquid crystal molecules.
For example, in a liquid crystal display device containing a liquid crystal material comprising a pleochroic dye with parallel dichroism and nematic liquid crystal compounds with positive dielectric anisotropy and having been subjected to homogeneous orientation treatment, the dye molecules form a homogeneous alignment wherein the longer axis of the dye molecule is aligned in parallel with electrode planes similarly with the liquid crystal molecules. When white light traveling in a perpendicular direction to the electrode planes and polarized to the same direction as the average direction of the oriented liquid crystal molecules travels through the liquid crystal layer in the above-described state, the electric vector of the light becomes parallel with the average direction of the dye molecules and, as a result, a particular wavelength region is strongly absorbed, thus the liquid crystal layer taking a strongly colored state. Application of a voltage to the liquid crystal layer in the above-described state causes homeotropic alignment of the longer axis of the dye molecule due to the positive dielectric anisotropy of the host liquid crystal. In this state, the average direction of the longer axes of the dye molecules becomes perpendicular to the electric vector of incident white light, and hence the incident light is scarcely absorbed by the dye molecule and, as a result, the liquid crystal layer takes a wealky colored state.
The guest-host effect can be obtained not only by utilizing nematic liquid crystal compounds as described above but by utilizing, for example, smectic liquid crystal compounds or utilizing cholesteric-nematic phase change as well. In the case of utilizing phase transition, each molecule of a cholesteric liquid crystals in a device having been subjected to homogeneous orientation treatment takes a helical arrangement (the helical axis perpendicular to the electrode plane). When white non-polarized light travels through the liquid crystal layer in a direction perpendicular to the electrode planes, a particular wavelength region of all planes of polarization of the light can be strongly absorbed by the dye molecules and, as a result, the liquid crystal layer appears strongly colores. When the host cholesteric liquid crystal have positive dielectric anisortopy, application of a voltage to the liquid crystal layer in the above-described state causes homeotropic alignment of the longer axis of the dye molecule following the liquid crystal molecule due to the relaxation of the helical arrangement of the liquid crystal molecules, thus the liquid crystal layer appearing weakly colored.
The pleochroic dyes as described above are required to possess: (1) a high "order parameter" (presented as S) in a host liquid crystal; (2) a sufficient solubility in a host liquid crystal; (3) a high stability against light, heat, and electricity; and (4) a hue according to the end-use. Of these factors, at least (1) and (2) are required for raising contrast of a resulting display device.
The order parameter S is defined by the equation (I) and is experimentally determined by the equation (II): EQU S=(3 cos.sup.2 .theta.-1)/2 (I) EQU S=(A.sub.// -A.perp.)/(2A.perp.+A.sub.//) (II)
wherein the term of cos.sup.2 .theta. is timewise averaged, .theta. represents an angle which the transition axis of the dye molecule makes with the average direction of host liquid crystal molecules, and A.sub.// and A.perp. represent the absorbances of the dye molecules for the light polarized parallel to and perpendicular to the average direction of the host liquid crystal, respectively.
With pleochroic dyes having parallel dichroism, the nearer the S value to 1 which is the theoretical maximum, the less the degree of residual color in a weakly colored state, which serves to realize high brightness, high contrast display.
An object of the present invention is to provide novel pleochroic dyes having parallel dichroism and capable of satisfying the aforesaid requirements (1), (2) and (4), a liquid crystal composition which can produce high contrast, high brightness display, and a display device containing the composition.
Relationship between the molecular structure of pleochroic dye and the various properties has not fully been clarified yet, and hence it is quite difficult to select a pleochroic dye which has a desired hue and satisfies all requirements described hereinbefore based on knowledges about known dyes.